1. Field of the Invention
The present invention relates to a reactive volt-ampere-hour meter and, more particularly, to a reactive volt-ampere-hour meter of the type electronically measuring the amount of reactive power of loads for AC applications.
2. Description of the Prior Art
Energy charge for home appliances is computed on the basis of active power indicated by an integrating wattmeter. However, when it comes to factories and other large users using many electric appliances including motors, it is necessary to measure not only the active power but also reactive power of such loads in computing energy charge. Specifically, it is a common practice to apply a so-called force rate article prescribing that the charge should be discounted when the force rate of loads is higher than a standard value or should be increased if the former is lower than the latter. For example, assuming that he standard force rate is 85 percent, the basic charge is discounted by 1 percent per 1 percent of the part exceeding 85 percent or should be increased by 1 percent per 1 percent of the part falling short of 85 percent. A large user, therefore, has to be furnished with a reactive volt-ampere-hour meter in addition to an integrating wattmeter, so that the reactive power of AC loads may be measured throughout the period in which power is used.
An integrating wattmeter for measuring active power is decreasing in size due to electronic implementations. For example, U.S. Pat. No. 4,992,725 issued to the same inventor as the present invention discloses a wattmeter having a pair of analog-to-digital (A/D) converters for converting respectively two analog signals representative of an AC voltage and an AC current fed to an AC load to digital signals, a multiplier for multiplying the digital signals, and an integrating unit for integrating the results of multiplications. The integrated value of the integrating unit indicates a measured amount of active power.
Theoretically, if a 90-degrees phase shift circuit precedes either of the two A/D converters of such an integrating wattmeter for shifting the phase of the input analog signal by 90 degrees, the integrated value of the integrating unit is expected to give the amount of reactive power of a load and, therefore, to readily implement a reactive volt-ampere-hour meter. However, this approach is not practical since the phase of a current to a load relative to the phase of a voltage changes with the connection of electrical equipment to be used and with the size of a load. Specifically, when the phase of a current to a load is retarded relative to the phase of a voltage (simply referred to as a retarded phase hereinafter), the meter integrates reactive power in the positive direction. Conversely, when the former is advanced relative to the latter (simply referred to as an advanced phase hereinafter), the meter integrates reactive power in the negative direction. Therefore, when use is made of only one reactive volt-ampere-hour meter having the above construction for measuring the amount of reactive power, a difference between the value integrated during the period of retarded phase and the value integrated during the period of advanced phase will be the result of measurement due to the retarded phase and advanced phase. Then, the output of the meter will be smaller than the actual value, resulting in the overestimation of the force ratio of the load.
In light of the above, it has been customary to connect two reactive volt-ampere-hour meters to the same load in parallel and assign one of them to retarded phase and the other to advanced phase. The absolute values of the results of measurement by the two meters are added to produce an actual amount of reactive power. In such a case, use is made of a reactive volt-ampere-hour meter of the type having an integrating unit made up of a cumulative adder, an up-down counter, and an up-counter and connected to the output terminal of the multiplier in place of the previously stated integrating unit which simply integrates the products from the multiplier. The cumulative adder has a predetermined number of figures and cumulatively adds products from the multiplier to produce a single carry pulse every time a positive overflow occurs or to produce a single borrow pulse every time a negative overflow occurs. The up-down counter up-counts the carry pulses from the cumulative adder and down-counts the borrow pulses to generate a single carry pulse every time a positive overflow occurs. The up-counter up-counts the carry pulses from the up-down counter. The number of carry pulses from the cumulative adder becomes greater than that of the borrow pulses when the amount of power is positive or becomes smaller than the latter when the amount of power is negative. The up-down counter up-counting the carry pulses and down-counting the borrow pulses generates carry pulses the number of which is proportional to the amount of power only when the amount is positive. Therefore, the count of the up-down counter indicates the positive amount of power. One of the two meters each being responsive to the positive amount of power only is provided with a 90-degrees phase shift circuit for shifting the phase of an analog signal representative of an AC current by 90 degrees, implementing a reactive volt-ampere-hour meter for advanced phase. The other meter is provided with a 90-degrees phase shift circuit for shifting the phase of an analog signal representative of an AC voltage by 90 degrees, constituting a reactive volt-ampere-hour meter for retarded phase. Since the former meter and the latter meter measure respectively only the amount of reactive power during the period of advanced phase and only the amount of reactive power during the period of retarded phase, adding the outputs of the two meters is successful in producing an actual amount of reactive power.
However, the dual reactive volt-ampere-hour meter scheme stated above is not desirable from the cost performance standpoint since it needs not only two such meters but also means for adding the outputs of the two meters.